The invention relates to data entry unit systems and more particularly to such a system in which a plurality of serially connected data entry units are arranged in a closed loop.
Closed loop data entry unit systems have been previously proposed and in order to maintain synchronism between a controller connected to the loop and the data entry units, extra leads have generally been provided, in addition to the required lead pair for transmitting data, for clocking the data entry units with respect to the controller so as to maintain synchronism between the data entry units and the controller. Since the prior data entry unit systems were not self-clocking, if no auxiliary clocking lead was desired, it was necessary to provide each of the data entry units with a local oscillator. Also, such prior data entry units required logic for determining if valid and complete data was received by each of the data entry units.